The present invention relates to papermaking, and more particularly to papermaking which yields paper having regions of different regions, such as density, caliper, crepe, amplitude, etc.
Papermaking is a well known art. In papermaking cellulosic fibers and a liquid carrier are mixed together. The liquid carrier is drained away and the resulting embryonic web of cellulosic fibers is dried.
Drying is typically accomplished in one of two manners, through air drying or conventional drying. Through air drying relies upon blowing hot air through the embryonic web. Conventional drying relies upon a press felt to remove water from the web by capillary action.
Through air drying yields paper having regions of different densities. This type of paper has been used in commercially successful products, such as Bounty paper towels and Charmin and Charmin Ultra brands of bath tissues. However, there are or may be situations where one does not wish to utilize through air drying.
In these situations, conventional felt drying is used. However, conventional felt drying does not necessarily produce the structured paper and its attendant advantages. Accordingly, it has been desired to produce structured paper using conventional felt drying. This has been accomplished utilizing a conventional felt having a patterned framework thereon for imprinting the embryonic web. Examples of these attempts in the art include commonly assigned U.S. Pat. No. 5,556,509, issued Sep. 17, 1996 to Trokhan et al.; U.S. Pat. No. 5,580,423, issued Dec. 3, 1996 to Ampulski et al.; U.S. Pat. No. 5,609,725, issued Mar. 11, 1997 to Phan; U.S. Pat. No. 5,629,052, issued May 13, 1997 to Trokhan et al.; U.S. Pat. No. 5,637,194, issued Jun. 10, 1997 to Ampulski et al.; U.S. Pat. No. 5,674,663, issued Oct. 7, 1997 to McFarland et al.; and U.S. Pat. No. 5,709,775 issued Jan. 20, 1998 to Trokhan et al., the disclosures of which are incorporated herein by reference.
However, there may be occasions when one wishes to use a conventional felt without a patterned framework thereon. Such flexibility in the manufacturing process is provided by the present invention.
U.S. Pat. No. 4,421,600 issued Dec. 20, 1983 to Hostetler discloses an apparatus having two felts, and three pressing operations. Again, after pressing the web is transferred on a woven imprinting fabric to the Yankee dryer.
Such an attempt in the art is illustrated by U.S. Pat. No. 4,309,246 issued Jan. 5, 1982 to Hulit et al. Hulit et al. describes three configurations where a nip is formed between two rolls. In each configuration, an imprinting fabric and a felt are interposed between the rolls. A paper web is carried on the imprinting fabric and imprinted by the imprinting fabric. Hulit then carries the web from this nip on the imprinting fabric. In two embodiments, Hulit then transfers the web from the imprinting fabric to a Yankee drying drum. In the third embodiment, Hulit does not use a Yankee drying drum.
The Hulit arrangement have several disadvantages. First, two sets of nips are required, a first nip to imprint the web and a second nip where the web is transferred to the Yankee drying drum, Hulit recognizes that dryer drums may be utilized instead of, or in addition to, the Yankee drying drum. However, Hulit does not minimize the expense and inconvenience of requiring two separate nips for the configurations relying upon the Yankee drying drumxe2x80x94as most commonly occurs in the art.
Furthermore, by imprinting the paper web in a first operation, and transferring the paper web to the Yankee drying drum in a second operation, Hulit creates caliper in one operation which he destroys in a subsequent operation. Compaction of Hulit""s web necessarily occurs between the compression roll and the Yankee dryer. This compaction destroys the caliper that was built in by imprinting the web in the first place.
Another attempt is shown in European Patent 0 526 592 B1 granted Apr. 5, 1995 to Erikson et al. Erikson et al. discloses another two nip configuration. In the first nip, the paper is imprinted between a the press roll and a lower press roll. There, Erikson et al. dewaters the paper by placing the press felt directly against the paper. This allows the press felt to deform into the areas of the imprinting fabric not supported by knuckles, reducing the differential density effects of the compaction caused by the imprinting fabric.
Erikson imprints the paper and transfers it to the Yankee at a lower press roll. The paper is transferred to the Yankee drying drum at this point. However, the second press drum again imprints the paper. The problem presented by the Erikson et al. is that its second nip the imprinting belt is never in registration with the imprinted pattern provided at the first nip. Thus, Erikson unduly compacts the paper and destroys the caliper it creates by imprinting at the first nip.
Furthermore, Erikson et al. like the aforementioned attempts in the art, still requires a complex two nip system. Erikson also requires the press felt loop to be outboard of the imprinting fabric loop. This arrangement creates a very expensive proposition for retrofit to existing machinery, as additional space, drives, etc. are required to add the separate felt loop. The cost of installing such a separate felt loop on an existing papermaking machine is estimated to be in excess of $10 million.
Commonly assigned U.S. Pat. No. 5,637,194 issued Jun. 10, 1997 to Ampulski et al., the disclosure of which is incorporated herein by reference, discloses an alternative paper machine embodiment where a felt is positioned adjacent a face of the imprinting member. The imprinting members imprints the molded web and carries it to the Yankee drying drum. Ampulski et al. teaches that the imprinting member is useable for through-air drying and the web is molded into the imprinting member. While Ampulski et al. represents a significant improvement over the prior art, Ampulski et al. still does not provide full contact of the web against the Yankee drying drum.
The invention comprises a papermaking apparatus. The apparatus has first and second rigid surfaces juxtaposed to form a nip therebetween. An imprinting member is interposable in the nip and carries a web. The imprinting member has a sheet side for contacting the web and an opposed backside. The web is in contacting relationship with the first rigid surface whereby the web can be simultaneously imprinted and transferred to the first rigid surface.
A capillary dewatering member is also interposable in the nip and is contacting relationship with the second rigid surface and with the backside of the imprinting member. A compressive force can be simultaneously applied to the web, the imprinting member and the capillary dewatering member through the nip.
In a preferred embodiment, the nip is formed by two coaxial rolls. One of the rolls may be a Yankee drying drum, the other roll may be a pressure roll and more preferably a vacuum pressure roll.